The present invention relates to a brush mechanism in a direct-current motor, and, more specifically, to a supporting structure for a brush mechanism.
FIG. 7 is a plan view illustrating a prior art direct-current motor. FIG. 8 is a cross-sectional view taken along line 8—8 of FIG. 7. The direct-current motor includes a brush mechanism 50. The brush mechanism includes a commutator 51, brushes 52 contacting the commutator 51, brush boxes 53 each supporting one of the brushes 52, and a substrate 54. The brush boxes 53 are fixed to the substrate 54. The substrate 54 is made of an insulating material such as resin, and is fixed to a motor housing 55. When the commutator 51 rotates, frictional force between the brushes 52 and the commutator 51 is unstable. Also, steps among the segments of the commutator 51 collide with the brushes 52. Accordingly, the brushes 52 generate self-excited vibration. The vibration is transmitted to the motor housing 55 through the substrate 54. The vibration is further transmitted to a motor yoke 56, which produces noise from the motor yoke 56.
In a motor used in a vehicle, such as a motor in an electric power steering, the substrate 54 is supported by the motor housing 55 with a rubber vibration absorbing member 57. This structure prevents vibration from being transmitted from the brushes 52 to the motor housing 55. As shown in FIG. 8 and 10, the substrate 54 is fixed to the motor hosing 55 with the two vibration absorbing members 57 and screws 58, which are located at positions K4, K5, respectively.
However, as shown in FIGS. 7 and 9, the positions K4, K5 are on a line L0 (the diameter line of the substrate 54), which is perpendicular to the central axis of the substrate 54. That is, the positions K4, K5 are spaced by 180° about the central axis of the substrate 54. Also, the position K4 is located between two terminals 60. Each terminal 60 is connected to a lead wire 59 for supplying electricity to one of the brushes 52. The terminals 60 are fixed to the substrate with the position K4 in between.
When an external force is applied to the lead wires 59, the force is transmitted to the substrate 54 through the terminals 60. Especially, when the load received by one of the lead wires 59 is greater than the load received by the other lead wire 59, the external force rotate (incline) the substrate 54 about the line L0, on which the positions K4, K5 are located. Therefore, the brushes 52, which are fixed to the substrate 54, are inclined with respect to the motor housing 55, which changes the contacting state of the brushes 52 and the commutator 51. This adversely affects the rectification of the motor and causes the motor to rotate at a nonuniform rotational speed. Further, it is difficult to effectively absorb vibration of the substrate 54 produced by external force at the only two positions K4, K5.